The inspection of leads of Integrated Circuit (IC) packages is extremely important to the electronics industry. IC packages such as Quad Flat Pack (QFP), Plastic Leadless Chip Carrier (PLCC), Small Outline IC (SOIC), Small Outline J-Lead (SOJ), Dual-In-Line Pack (DIP), Small Outline Transistor (SOT), and their derivatives have leads protruding out from the IC package body. These leads are the means for electrically connecting the internal circuits to the outside world. The integrity of these leads is crucial for providing good electrical connectivity and therefore useful application of the IC.
An Integrated Circuit (IC) package typically has a square or rectangular plastic package moulded over and encapsulating the IC circuitry commonly known as the "die". The size of the package may range from 4.times.4 mm square to 32.times.32 mm square. Extending from the plastic package are leads that provide electrical connectivity between the die inside the IC package and the printed circuit boards (PCBs). It is important for IC packages and their leads to possess accurate and consistent mechanical dimensions because highly automated PCB assembly machines are used to place and solder the ICs onto PCBs. Damaged, twisted, or out-of-place leads will likely lead to improper assembling of the PCB assembly, and hence the malfunction of the PCB assembly. In particular, for high lead count ICs such as the Quad Flat Pack (QFP) which has leads on all four sides of the package, the mechanical requirements are even more stringent. There are several requirements of the leads and categories of defects that have to be measured, the leads requirement including coplanarity, lead pitch, terminal dimension, standoff, and others. Lead defects include bent leads, solder plating defects, swept leads, burr, and the like.
Several techniques and systems that include special optical and lighting arrangements for the direct and indirect viewing of leads of IC packages are available on the market. There are two major categories of IC lead inspection and measurement systems. One category involves the use of laser-scanning techniques. The other category involves the use of shadow casting and backlighting techniques to illuminate the lead profiles onto imaging planes. As an example, one method uses lasers to scan the leads of an IC from the top. Another method uses a backlighting system with an image doubler that increases the resolution of the image. Yet another method uses a lead inspection system to locate the leads with reference to a reference plate on which the device is mounted and a real-time reference which is used to provide a known correlation between the image pixels and linear measurement. One such IC inspection system includes a displacement sensor in which the upward and downward coplanarity error of each lead is measured from a level change in the output signal of the sensor. Another such system determines a position of at least one lead of an electronic component using shadow casting techniques.
The way that the leads are arranged on an IC also determines how the IC will be inspected using the IC inspection systems. This is because ICs with different lead arrangements are transported differently. For example, an IC having leads on all four sides of its package is typically transported by pick-and-place means such as pickup heads. Since these pickup heads move through a range of motion, these IC inspection systems must therefore be designed to not obstruct or impede this range of motion. Whereas in another example, an IC having leads on only two opposing sides of its package is typically transported using a track. Most dual sided ICs such as SOICs, SOJs and DIPs are transported using tracks during electrical testing and visual mechanical inspection. Single sided ICs are also transported for testing and inspection via tracks.
Two types of track feed methods are available. The gravity-feed method uses inclined tracks in which the ICs are loaded from the top of the track at one place and they slide down the track to another place. The force-feed method depends on pushers to push the ICs along a horizontal guided track. Again, the IC inspection systems for dual-sided or single-sided ICs must be adapted to the tracks that transport them.
However, various problems exist with these prior art inspection techniques. In the case of laser scanning technique, the top surface instead of the bottom surface of a lead is measured. This presents a problem because the bottom surface and geometry of a lead is more important than the top surface, particularly in relation to the electrical connectivity of the leads. The thickness of the leads will vary from lead to lead as a result of the solder plating thickness. As a consequence, measuring the top surface of the leads is not as effective to measuring the bottom surface. This is especially true in high precision measurement in the range of several micrometers. In addition, laser-scanning techniques cannot detect burrs on lead tips, which commonly occur as a result of the trim and form process in the manufacture of IC leads. The existence of burrs on lead tips is another important factor that influences the electrical connectivity of an IC package to PCB.
In particular, problems exist in prior art inspection systems for dual-sided ICs transported using tracks. These tracks are typically continuous for the distance that they transport the dual-sided ICs, and thus will impede the imaging of the profiles of the leads in shadow casting and backlighting techniques. Therefore, a prior art solution uses an illuminating track as a backlight. The biggest difficulty encountered in such an implementation is the mounting of the backlight within the track without adversely affecting the speed with which the ICs are transported.
Therefore, there is a need for methods and systems for reliable inspection of ICs, especially for a method and a system that can reliably inspect the bottom surface of the IC leads.